A motorcycle radiator filler cap is a component that is used to add coolant to the radiator system. The radiator filler cap is located on top of the filler and serves as a valve to release pressure and prevent coolant from spilling out of the system. The filler cap also helps maintain pressure in the radiator, ensuring proper coolant circulation and helping to keep the engine at the right temperature. It is important to use the correct type of filler cap and ensure that it is properly tightened to avoid leaks and ensure that the radiator system functions properly.